Deep well earthboring for gas, crude petroleum, minerals and even water or steam requires tubes of massive size and wall thickness. Tubular drill strings may be suspended into a borehole that penetrates the earth's crust several miles beneath the drilling platform at the earth's surface. To further complicate matters, the borehole may be turned to a more horizontal course to follow a stratification plane.
The operational circumstances of such industrial enterprise occasionally present a driller with a catastrophe that requires him to sever his pipe string at a point deep within the wellbore. For example, a great length of wellbore sidewall may collapse against a drill string and cause the drill string to wedge tightly in the well bore. Thereafter, the wedged drill string cannot be pulled from the well bore and, in many cases, cannot even be rotated. A typical response for salvaging the borehole investment is to sever the drill string above the obstruction, withdraw the freed drill string above the obstruction, and return to the wellbore with a “fishing” tool to free and remove the wedged portion of the drill string.
The drill string weight, which is bearing on the drill bit and necessary for advancement into the earth strata, is provided by a plurality of specialty pipe joints having atypically thick annular walls. In the industry vernacular, these specialty pipe joints are characterized as “drill collars.” A drill control objective is to support the drill string above the drill collars in tension. Theoretically, only the weight of the drill collars bears compressively on the drill bit. With a downhole drilling motor, which is configured for deviated bore hole drilling, the drill motor, bent sub and drill bit are positioned below the drill collars. This drill string configuration does not rotate in the borehole above the drill bit. Consequently, the drill collar section of the drill string is particularly susceptible to borehole seizures and because of the drill collar wall thickness, is also difficult to cut.
When an operational event, such as a “stuck” drill string, occurs, the driller may use wireline suspended instrumentation that is lowered within the central, drill pipe flow bore to locate and measure the depth position of the obstruction. This information may be used to thereafter position an explosive severing tool within the drill pipe flow bore.
Typically, an explosive drill pipe severing tool comprises a significant quantity, 800 to 1,500 grams (12,345 grains to 23,149 grains) for example, of high order explosive, such as RDX, HMX or HNS. The explosive powder is compacted into high density “pellets” of about 22.7 grams to about 38 grams (350 grains to 586 grains) each. The pellet density is compacted to about 1.6 gm./cm3 to about 1.65 gm./cm3 (404.6 grains/inch3 to 417.3 grains/inch3) to achieve a shock wave velocity greater than about 9144 meters/second (30,000 ft/sec), for example. A shock wave of such magnitude provides a pulse of pressure in the order of 2.8×104 MPa (4×106 psi). It is the pressure pulse that severs the pipe.
In one form, the pellets are compacted, at a production facility, into a cylindrical shape for serial, juxtaposed loading at the jobsite as a column in a cylindrical barrel of a tool cartridge. Due to weight variations within an acceptable range of tolerance between individual pellets, the axial length of explosive pellets fluctuates within a known tolerance range.
Extreme well depth is often accompanied by extreme hydrostatic pressure. Hence, execution of the drill string severing operation may be required at hydrostatic pressures above 206.94 MPa (30,000 psi). Such high hydrostatic pressures tend to attenuate and suppress the pressure of an explosive pulse to such degree as to prevent separation.
One prior effort, by the industry, to enhance the pipe severing pressure pulse and to overcome high hydrostatic pressure suppression has been to detonate the explosive pellet column at both ends simultaneously. Theoretically, simultaneous detonations at opposite ends of the pellet column will provide a shock front from one end colliding with the shock front from the opposite end within the pellet column at the center of the column length. On collision, the pressure is multiplied, at the point of collision, by about 4 to 5 times the normal pressure cited above. To achieve this result, however, the detonation process, particularly the simultaneous firing of the detonators, must be timed precisely in order to assure collision at the center of the explosive column.
Such precise timing is typically provided by means of mild detonating fuse and special boosters. However, if fuse length is not accurately cut or problems exist in the booster/detonator connections, the collision may not be realized at all and the device will operate as a “non-colliding” tool with substantially reduced severing pressures.
The reliability of state-of-the-art severing tools is further compromised by complex assembly and arming procedures required at the well site. With those designs, laws and regulations require that explosive components (detonator, pellets, etc.) must be shipped separately from the tool body. Complete assembly must then take place at the well site under often unfavorable working conditions.
Finally, the electric detonators utilized by many state-of-the-art severing tools are vulnerable to stray electric currents and uncontrolled radio frequency (RF) energy sources, thereby further complicating the safety procedures that must be observed at the well site.